Suppose you created an LVM mirror but your mirror log is on the wrong SAN device. SAN migration won't help you since these LUNs exist on different SAN machines altogehter. But how do you move the mirror log?
Let's create an LVM mirror:
Last login: Sat Nov 23 17:36:23 2013
[root@leonis ~]# ls /dev/sd*
/dev/sda /dev/sdb
[root@leonis ~]# pvcreate /dev/sda
Writing physical volume data to disk "/dev/sda"
Physical volume "/dev/sda" successfully created
[root@leonis ~]# pvcreate /dev/sdb
Writing physical volume data to disk "/dev/sdb"
Physical volume "/dev/sdb" successfully created
[root@leonis ~]# vgcreate vgmir /dev/sda /dev/sdb
Volume group "vgmir" successfully created
[root@leonis ~]# lvcreate -l 45%FREE vgmir -n lvmir
Logical volume "lvmir" created
[root@leonis ~]# lvconvert -m 1 /dev/vgmir/lvmir
vgmir/lvmir: Converted: 0.2%
vgmir/lvmir: Converted: 2.2%
...
vgmir/lvmir: Converted: 99.5%
vgmir/lvmir: Converted: 100.0%
Now lvs -a -o +devices will show you the detailed mirror configuration:
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert Devices
lvmir vgmir mwi-a-m- 3.59g lvmir_mlog 100.00 lvmir_mimage_0(0),lvmir_mimage_1(0)
[lvmir_mimage_0] vgmir iwi-aom- 3.59g /dev/sda(0)
[lvmir_mimage_1] vgmir iwi-aom- 3.59g /dev/sdb(0)
[lvmir_mlog] vgmir lwi-aom- 4.00m /dev/sdb(920)
In my case /dev/sdb and /dev/sdc were on different SAN machines and one machine was loaded quite heavily. Meaning we got a 160 IOPS maximum out of the entire mirror. Now all credit goes to our SAN admins for getting the utilization of the older SAN machine to the point where the LUN could easily deliver 4000 IOPS.
However there's one thing to consider: the mirror log. The mirror log generally incurs extra IOPS, so you want to create it on the faster/less loaded SAN machine. Two approaches immediately come to mind: a) convert the mirror from on disk log to in-memory then recreating it on the faster SAN machine; b) dropping the slower mirror leg entirely and recreating the mirror with the faster SAN as the mirror device.
Let's drop the mirror:
[root@leonis ~]# lvconvert -m 0 vgmir/lvmir /dev/sdb
Logical volume lvmir converted.
And recreate the mirror with the mirror log on ``/dev/sda``::
[root@leonis ~]# lvs -a -o +devices,seg_pe_ranges
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert Devices PE Ranges
lvmir vgmir -wi-a--- 3.59g /dev/sda(0) /dev/sda:0-919
[root@leonis ~]# lvconvert -m 1 vgmir/lvmir /dev/sdb:0-919 /dev/sda
vgmir/lvmir: Converted: 0.2%
vgmir/lvmir: Converted: 2.7%
vgmir/lvmir: Converted: 99.8%
vgmir/lvmir: Converted: 100.0%
[root@leonis ~]# lvs -a -o +devices
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert Devices
lv_root VolGroup -wi-ao-- 3.45g /dev/vda2(0)
lv_swap VolGroup -wi-ao-- 3.88g /dev/vda2(882)
lvmir vgmir mwi-a-m- 3.59g lvmir_mlog 100.00 lvmir_mimage_0(0),lvmir_mimage_1(0)
[lvmir_mimage_0] vgmir iwi-aom- 3.59g /dev/sda(0)
[lvmir_mimage_1] vgmir iwi-aom- 3.59g /dev/sdb(0)
[lvmir_mlog] vgmir lwi-aom- 4.00m /dev/sda(920)
The trick here is, since we don't have an option to specify the mirror leg explicitely we specify a PE range on /dev/sdb and the full /dev/sda and LVM figures out where to place the segments.
Let's convert that to a core mirror:
[root@leonis ~]# lvconvert --mirrorlog core vgmir/lvmir
Logical volume lvmir converted.
[root@leonis ~]# lvs -a -o +devices
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert Devices
lv_root VolGroup -wi-ao-- 3.45g /dev/vda2(0)
lv_swap VolGroup -wi-ao-- 3.88g /dev/vda2(882)
lvmir vgmir mwi-a-m- 3.59g 100.00 lvmir_mimage_0(0),lvmir_mimage_1(0)
[lvmir_mimage_0] vgmir iwi-aom- 3.59g /dev/sda(0)
[lvmir_mimage_1] vgmir iwi-aom- 3.59g /dev/sdb(0)
Then convert it back to a on-disk mirror::
[root@leonis ~]# lvconvert --mirrorlog disk vgmir/lvmir /dev/sdb
Logical volume lvmir converted.
[root@leonis ~]# lvs -a -o +devices
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert Devices
lv_root VolGroup -wi-ao-- 3.45g /dev/vda2(0)
lv_swap VolGroup -wi-ao-- 3.88g /dev/vda2(882)
lvmir vgmir mwi-a-m- 3.59g lvmir_mlog 100.00 lvmir_mimage_0(0),lvmir_mimage_1(0)
[lvmir_mimage_0] vgmir iwi-aom- 3.59g /dev/sda(0)
[lvmir_mimage_1] vgmir iwi-aom- 3.59g /dev/sdb(0)
[lvmir_mlog] vgmir lwi-aom- 4.00m /dev/sdb(920)
Converting the mirror-log to a core-log is clearly much faster and the only drawback seems to be that in case of a reboot the whole mirror will be resynced which is just as good as resyncing the mirror in the first place.